Coherent optical communication systems have been known as high-speed, large-capacity optical communication systems. An optical receiver of a coherent optical communication system separates signal light inputted from a single-mode optical fiber into X-polarized light and Y-polarized light orthogonal to each other by using a polarization beam splitter (PBS). Thereafter, an optical hybrid makes the signal light interfere with local oscillator light (LO light) inputted from a polarization-maintaining optical fiber. Thereafter, a light-receiving element (e.g., photodiode (PD)) arranged dower stream of the optical hybrid converts an optical signal into an electric signal.
Known as a method for aligning an optical axis of the polarization-maintaining optical fiber is one comprising inserting a polarizer between the light-receiving element and the polarization-maintaining optical fiber and adjusting an angle of the polarization-maintaining optical fiber such that the light-receiving element attains the maximum output current in this state. For suppressing light reflection, an end face of the polarization-maintaining optical fiber may form a predetermined angle (e.g. about 8°) with a plane perpendicular to the optical axis.